Article pubs.acs.org/JPCC
Size- and Shape-Controlled Electrochemical Deposition of Metal Nanoparticles by Tapping Mode Atomic Force Microscopy Ichiro Tanabe and Tetsu Tatsuma* Institute of Industrial Science, University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan ABSTRACT: An electrochemical method for fabrication of metal nanoparticles with controlled size and shape was developed. Ag ions are adsorbed on a conductive substrate, and a voltage is applied between the substrate and a tip of atomic force microscopy in contact mode or tapping mode for deposition of Ag nanoparticles. Although control of particle size is difficult in contact mode because of deterioration in cantilever elasticity, it is possible in tapping mode through regulating the applied voltage or voltage application time. The present method can also be applied to fabrication of Ag nanorods and Ag nanoparticle arrays with controlled size and spacing and deposition of Au nanoparticles.
1. INTRODUCTION Metal nanoparticles have attracted much attention as materials with interesting optical, electrochemical, magnetic, and catalytic functionalities.1−4 In particular, nanoparticles of noble metals such as Au and Ag absorb and scatter light based on localized surface plasmon resonance (LSPR) at specific wavelengths that depend on the dielectric environment of the nanoparticles.5,6 Au and Ag nanoparticles are therefore applied to chemical sensing3 and biosensing.7 Additionally, localization of oscillating electric fields (optical near fields) in the vicinity of the nanoparticles8 is exploited for surface enhanced Raman scattering (SERS),3,9,10 fluorescence enhancement,11,12 photocurrent enhancement,13,14 and nanoimaging.15,16 We have found plasmon-induced charge separation at the interface between a Au or Ag nanoparticle and TiO2,17−19 which is applied to photocatalysis,18 photovoltaic cells,18−20 multicolor photochromism,17,21 surface patterning,22 and photomorphing gels.23 The LSPR wavelength and intensity as well as localization of electric fields strongly depend on the metal species,24,25 particle size,24−26 shape,24,26 and orientation27,28 of the nanoparticle, and interparticle distance.27,29 Control of these factors would therefore allow tuning of the abovedescribed functionalities and facilitate mechanistic studies of LSPR-based phenomena. Particle size and shape can be controlled by colloidal synthesis, but the synthesized nanoparticles are protected by ligands, which must be removed in catalytic and electrochemical applications. Moreover, it is difficult to control the particle orientation and interparticle distance on solid substrates. Electron beam lithography is a powerful method to control the geometry of metal nanoparticles on a solid substrate with a resolution of about 10 nm.30 However, this method needs large and expensive equipment and it requires coating and lifting-off of a photoresist. There are more convenient techniques with the use of atomic force microscopy (AFM) or scanning tunneling microscopy (STM), which would © 2012 American Chemical Society
be useful for optimization of particle properties and mechanistic studies. The field evaporation31 and field-enhanced atom transfer32,33 methods have been employed to fabricate metal nanostructures on an electroconductive substrate by applying voltage pulses between the substrate and a tip of AFM or STM. These methods allow precise control of the location and amount of metal deposition. However, since the metal atoms are transferred from the tip, the amount of metal deposition is limited by the amount of metal preloaded on the tip. Forouzan and Bard34 have developed an electrochemical method for deposition of Ag nanoparticles on the basis of contact mode AFM. Ag+ ions are preadsorbed on an electroconductive substrate, and an appropriate dc voltage is applied in air between the substrate and an electroconductive AFM tip. Although this method is convenient, control of particle size and shape and deposition of other metals have not yet been reported. In this work, we developed an electrochemical nanodeposition coupled with tapping mode AFM, which achieved better controllability of metal particle geometry in comparison with the conventional method coupled with contact mode AFM. The present method can be applied to fabrication of both Ag and Au nanostructures.
2. EXPERIMENTAL SECTION We used 0.05% Nb-doped conductive rutile TiO2(100) single crystal (Nb-TiO2, 5 Ω cm, Shinkosha) as a substrate. After pretreatment with 12 M aqueous HF for 10 min and annealing at 900 °C for 1 h, the substrate was immersed in 1 M aqueous NaOH overnight and left in pure water under black light for >12 h. The substrate thus cleaned was immersed in 0.01−5 M AgNO3 aqueous solution in the dark for 0.5 h so as to adsorb Received: December 21, 2011 Revised: January 13, 2012 Published: January 19, 2012 3995
dx.doi.org/10.1021/jp2123086 | J. Phys. Chem. C 2012, 116, 3995−3999
The Journal of Physical Chemistry C
Article
Figure 1. The mechanism of electrochemical deposition of Ag nanoparticles.
Figure 2. AFM images of Ag nanoparticles deposited in contact mode at ∼65% RH. Ag+ ions were adsorbed from 10 mM AgNO3.
Ag+ ions onto the TiO2 surface. The sample was rinsed thoroughly with pure water and fixed on the sample stage of the AFM (E-sweep/NanoNavi Station, SII Nanotechnology) with a conductive carbon adhesive tape (Nisshin EM), under UV cutoff fluorescent lamps (≥440 nm). The dark sample chamber was filled with N2 gas at relative humidity (RH) ∼ 65% unless otherwise noted. In contact mode measurements, we used an Rh-coated silicon cantilever (SI-DF3-R, spring constant = 1.6 N m−1, curvature radius = 10 nm, SII Nanotechnology). The sample was approached to the cantilever until the tip touched the sample surface without deflection of the cantilever. The sample was then pushed down by Δh (nm), and potential Es (V) was applied to the sample stage relative to the cantilever for T (s). After potential application was completed, the morphology of the sample surface was imaged at 1000 particles in 100 μm × 100 μm) was subjected to X-ray diffraction measurements. As a result, the main peak attributed to Ag(111) was observed and no other peaks attributed to Ag2O or AgO were detected (Figure 3a). From these results, we conclude that the deposited nanoparticles are Ag. The height of the deposited crystals is estimated to be about 30 nm by the Scherrer equation from the full width at half-maximum of the X-ray diffraction peak. On the other hand, the lateral diameter and height of the deposited nanoparticles were estimated from AFM images to be 100 ± 40 and 20 ± 15 nm (n = 30), respectively.
Since no nanoparticle was deposited at
